devDependencies are installed by default

[SCKevinO]

Describe the bug
The documentation (https://doc.scalingo.com/languages/nodejs/start#install-devdependencies) say that devDependencies are not installed by default unless the NPM_CONFIG_PRODUCTION environment variable is set to false. However, they seem to be installed anyway if the env var is not defined at all.

To Reproduce
Steps to reproduce the behavior:

1. Create a new app on Scalingo
2. Deploy a nodejs app containing devDependencies in their package.json
3. Check the deploy log to see that devDependencies were installed
4. Connect to the app with a one-off
5. Notice that the devDependencies are present in the image

[bbougon]

Hello,

to give more insights, the dev dependencies are pruned at the end of the installation.
Nevertheless, if you have a devdependency, let's say storybook, which depends on playwright, which install firefox, chrome and webkit, the prune does not cleanup the /app/.cache that contains the installed software.

Also, my 2 cents, allowing dev dependencies to be installed when NODE_ENV=production should be avoid